1. Field of the Invention
The present invention relates to an apparatus that obtains biometric information, such as a fingerprint, palm print, blood vessel arrangement, and so on, for user authentication. More precisely, the invention relates to a biometric information obtaining apparatus with a sweep-type fingerprint sensor, which apparatus obtains a series of partial images (fingerprint images) of a finger while it is moving with respect to the sensor surface (image-obtaining surface).
2. Description of the Related Art
With recent improvements in performance of compact information equipment, such as mobile phones and PDAs (Personal Digital Assistants), such equipment has increasingly been used to access communication networks and to store great amounts of user information therein, so that the need for improving the security performance of the equipment has been strongly emphasized.
For attaining a high level of security of such equipment, it is conceivable to employ a previous user authentication system that uses a password and an ID (Identification) having been commonly used. However, because of the problem that passwords and ID cards are susceptible to theft, a user authentication (verifying a current user of equipment as a previously registered user) system with higher security has been desired. To meet this demand, user authentication by human body part characteristics (biometric information) is considered a good method with high security, and in particular, fingerprint verification is advantageous in user convenience.
For fingerprint verification, an electrostatic fingerprint sensor or an optical fingerprint sensor is used to obtain a fingerprint (a pattern made of ridges that contact with the sensor surface and ditches that do not contact therewith) from a fingertip of a user who is to be identified. From a foreground image (for example, an image of ridges) of the fingerprint, its minutiae information (for example, information of branch points and end points of ridges) is extracted to be compared with registration minutia information that is previously obtained and registered as reference information for verification. In this manner, a user is identified, that is, user authentication is performed.
Generally speaking, a common type of fingerprint sensor (hereinafter sometimes called a flat type fingerprint sensor) for obtaining a fingerprint image from a user to be identified, is normally equipped with a sensor surface (image-obtaining surface) larger than the size of a human fingertip. Recently, however, for the purpose of introducing a fingerprint sensor in compact information equipment, such as a mobile phone or a PDA, the sensor surface is down-sized to be smaller than a human fingertip, and a sequence of partial images of a fingerprint are obtained through the sensor surface and then combined to regenerate the whole image of the fingerprint.
A sweep-type fingerprint sensor is one of the fingerprint sensors ready for such a recent situation (for example, refer to the following patent applications 1 and 2). The sweep-type fingerprint sensor has a small-sized rectangular image-obtaining surface (sensor surface/image capture surface) having a length sufficiently shorter than that of a human fingertip. While a finger is moving with respect to the image-obtaining surface or while the image-obtaining surface (fingerprint sensor) is moving with respect to the finger, the fingerprint sensor obtains a series of partial images of the fingerprint, based on which the whole image thereof is then reconstructed. From the thus-reconstructed fingerprint image, information of fingerprint minutiae (branch points and end points of ridges) is extracted/generated, and on the basis of the information, user authentication is performed. Here, note that such a relative movement between a finger and an image-obtaining surface is called a “sweep.”
Such a sweep-type fingerprint sensor has a problem of deteriorating verification performance, which is more frequent in this type of sensor than in a flat type sensor, because finger movement with respect to the sensor surface can distort the surface skin of the finger or reduce the area of a fingerprint image obtained. Accordingly, most of the users need to learn and practice how to move their fingers, resulting in lowered user convenience in comparison with a flat type fingerprint sensor. In order to learn appropriate finger movement, it is essential for a user to be aware of his finger's current movement.
Both of the techniques disclosed in the following patent applications 1 and 2, however, delete information about finger movement, without offering a user any of the information. They show only a reconstructed image of a finger after the finger finishes with its movement or a verification result alone. Therefore, if failure in reconstruction of a fingerprint image or in fingerprint verification is caused by inappropriate finger movement, a user is not notified about the cause of the failure.
In the meantime, the following patent applications 3 and 4, for example, disclose techniques of guiding a user to put his finger at a correct position on a flat type fingerprint sensor. These techniques aim at detecting a positional relationship between the center of a sensor and a finger so as to lead the finger into the sensor center. Therefore, even if these techniques are applied to a sweep-type fingerprint sensor, it is still impossible to detect what causes distortion of a fingerprint image, so that a user still cannot recognize the fact that the distorted fingerprint image (that is, failure in fingerprint image reconstruction and in fingerprint verification) results from his erroneous finger movement. A flat type fingerprint sensor obtains such a fingerprint image as is shown in FIG. 32B, under a condition where a finger 101 is laid still on a sensor surface (image-obtaining surface) 100 as shown in FIG. 32A. Accordingly, the flat type sensor needs an instruction as to where a finger 101 should be placed with respect to the sensor surface 100, but has no need at all for real-time display/instruction of movement of a finger 101.
[Patent application 1] Japanese Patent Application Laid-open No. HEI 10-091769
[Patent application 2] Japanese Patent Application Laid-open No. HEI 11-253428
[Patent application 3] Japanese Patent Application Laid-open No. 2002-288643
[Patent application 4] Japanese Patent Application Laid-open No. 2002-177623
Against the background of the above patent applications 1-4, a technique has been desired whereby a user of a sweep-type fingerprint sensor can recognize inappropriate finger movement which causes distorted fingerprint images (that is, failure in fingerprint image reconstruction and in fingerprint verification).